The present invention relates to leak detection and determining leak location. To assist in understanding the present invention, problems related to detecting and locating air leaks in spacecraft are described. It should be understood, however, that the present invention can be used for detection of leaks and location of leaks in any number of other types of vessels, particularly pressure vessels, and is in no way limited to spacecraft.
It has been well known for a long time that orbital objects, such as meteoroids and space debris, are among the serious, but inevitable, threats to spacecraft intended for prolonged habitation. Technically, collisions with objects larger than 10-cm can be avoided using databases compiled by debris-tracking systems, such as the Haystack orbital debris radars and optical telescopes [1,2]. The strikes of small objects between 0.10 and 10.0 mm cannot, however, be avoided using the current devices. Furthermore, because the objects strike the spacecraft at high speeds, up to 10 to 15 km/s [3], even small particles in the size range of 1 mm could result in penetration of the pressure vessel and subsequent loss of air.
Referring to several reports [4-7], the major sources of the small particles are fragments of payloads, paint flakes, solid rocket motor slag and discharge, exploded rocket bodies, and extraterrestrial micrometeorites. Clearly, there will a higher probability of collisions with smaller objects if space exploration includes trips with longer operational times.
An example of a current long-endurance spacecraft is the International Space Station (ISS), now orbiting in low Earth orbit (LEO) with an altitude between 360 km and 440 km with an inclination of 51.6 degrees. It is designed to provide an Earth orbiting facility in order to develop advanced technology for human and robotic exploration of space. Ever since the ISS has launched in 1998, there have been many component replacements, which are mainly caused by the impact of high-speed micrometeorites and space resident debris [8]. In particular, the impacts of small objects cause air leaks when they strike a pressurized module on the ISS. As reported by NASA in January 2004 [9], parts of the ISS were nearly closed off for three weeks due to the air leaks. Although the size of the air leak was too small to endanger the crew and mission immediately, it was a time-consuming process to find and seal the leaks.
Accordingly, immediate location of the source of the leaks on the ISS is essential to provide the crew with maximum safety, minimize unnecessary mission effort, and maintain the operational status. By doing so, the crew can perform increasingly ambitious mission goals for longer periods with less ground supports. However, regardless of size of the air leaks, the detection and location of air leaks is a very difficult and time-consuming process because the size of an ISS module and the complexity of its construction. The crew cannot inspect the entire series of modules immediately. In addition, the ultrasound noise generated by escaping air at supersonic speed from small holes into the vacuum of space is not audible or detectable inside the ISS because of the nature of the leaks. Leaks on spacecraft occur from one atmosphere into vacuum and produce almost none of the characteristic detectable high-frequency sound, such as that typically exploited by industrial leak detection apparatus.
Conventionally, the crew on the ISS follows the sequential module leak isolation process [10]. The process involves very inefficient and time-consuming tasks, where the crew monitors the pressure difference while sequentially closing each hatch. Because it is not easy to discern very small pressure differences, the isolation process still does not reduce the leak detection time and risk of the crew's safety. Compared to the ISS, the MIR, a Russian space station, employs airflow induction sensors installed in the hatchways to continually monitor airflow and its rate of change [11]. Since the airflow induction sensors are designed to detect the very small changes in the airflow, they are also sensitive to the circulating air in a module. So, the crew must stop all the venting systems in a suspicious module to find air-leaks, and this action could cause more serious problems. In addition, pinpointing a leak location is not possible because the sensors are designed only to localize leaking to a specific module losing a measurable amount of air, and not to find its exact location within the module.
With the demands of more efficient leak detection systems on the ISS, active research has been performed to develop air leak detection systems. In 2000, Russian engineers have put their best efforts into an acoustic monitor called “OT 2-K” to detect air leaks on the MIR [12]. This sensor directs possible leak locations by measuring the noise level generated by escaping air. In the lab test, the sensor can detect the airborne noise in air escaping from a surgical needle at a distance of approximately 5 m. However, due to the nature of its working principle, it is not an efficient detector or locator of leaks inside a module. Instead, the crew must use the sensor outside of the module, an extremely time-consuming and often dangerous activity. Semkin et. al [13] have proposed a multiparameter transducer, which consists of one ionization sensor, four piezoelectric microphones, two thermocouple sensors, and a data processing unit in a single package. The dimensions of the transducer are 80×100×80 mm, and its weight is less than 0.35 kg. Analyzing the measurements obtained by each sensor, the transducer is capable of detecting gas leaks on the MIR from the holes with 0.1˜5 mm in diameter at a distance of 1.5 m from the transducer. However, the detectable zone is very small and restricted so that it is not a fully functional sensor to locate air leaks on a large-scale space station.
Recently, a company, called CTRL Systems, Inc., has developed and tested an ultrasonic leak detector (UL 101®) to locate air leaks on the ISS [14]. The detector is a hand-held, non-destructive diagnostic tool, which detects ultrasound in a narrow frequency band around 40 kHz. The outputs are supplied to a headset as well as to an analog meter. This method, however, is based on the same principle as most of the other industrial leak detectors, and is also not efficient for locating air leaks on the ISS because it was originally designed to detect leaks from pressurized vessels, where air is escaping into an atmospheric environment of 1 bar from a pressurized vessel at a pressure of at least 1 bar gage, or 2 bar absolute. On the contrary, the air leaks on the ISS are leaks into vacuum, where the direction of the escaping airflow is into the vacuum of space. Corsaro et al. [15] have proposed a prototype system of an acoustic particle impact detector, called “Particle Impact Noise-Detection and Ranging On Autonomous Platform (PINDROP)”. The PINDROP is designed to detect hypervelocity impacts by small particles and locate the impact sites using the propagation characteristics of the acoustic wave generated on a panel. The detector consists of a conventional aerogel particle-capture array to collect and characterize the small particles in space, acoustic sensor to locate their impact sites and autonomous data acquisition unit to process the collected data. As a active material in an acoustic sensor, poly-vanylidene fluoride (PVDF) is selected, owing to its unique advantages over other piezoelectric materials, such as high sensitivity in response to sudden changes in strain, very low mass, and flexible installation. To predict accurately the signal arrival time, an acoustic propagation model is developed and embedded in the data acquisition unit. This model is used to locate the impact sites using the relative time-of-arrival at three or more sensor locations. Although this detector is not designed to find the exact location of air leaks on the ISS, it provides the crew with valuable information when they are tracking down suspicious leak locations.
Kim et. al [11, 16] have developed a leak localization software for the ISS. The software uses the measurements from pressure gages, spacecraft attitude and rate sensors. When air leaks occur, the vent thrust generates a small torque on the space station. To preserve the spacecraft's attitude, a reaction torque is needed to stabilize the ISS, depending on the size and location of the leak. The software infers from the dynamics of the ISS stabilization, considering the geometrical structure of the ISS, a probable leak location and the estimated hole size. According to the developers, their software can locate holes with a diameter as small as 0.4 inches (>>1 cm). With its outstanding functionalities, NASA is planning to employ the software system on ISS in the near future [17]. In order to detect and predict the location of air leaks accurately, however, exact knowledge of the ISS geometric structure is required. If the spacecraft geometry changes, such as through the addition of more modules or docking with other spacecraft, the software must be entirely reconfigured to account for those variations in the spacecraft moment of inertia and mass.
In addition to developing the complex leak detection systems, it is desired to utilize the structure-borne noise so that the location of a leak is detected by remotely positioned sensors. One of the simple and widely used approaches is to use cross correlation. The technique assumes in the analysis that received signals by sensors at remote locations satisfy the conditions of a single mode of propagation with a non-dispersive (or frequency-independent) wave speed. Under the assumptions, the received signals are simply non-dispersed time-delayed replica of the signal generated at the leak. The time delay corresponds to the distance between the sensors and the leak source. They are, however, time-shifted with different amplitude because of differences in propagation paths of the signals. The location of peak on a time scale in the cross correlation function corresponds to the time delay, and, through the speed of sound in the structure, the location of the leak.
In 1969, White [18] first used a cross-correlation technique to determine a leak location on a thin plate, assuming that the structure-borne noise is propagating in all directions with the same propagation speed. The temporal shape of the measurements is also assumed to be unchanging with distance. However, the results of the cross correlation do not clearly show a large peak. Instead, it shows a gradual rise and fall of sinusoidal function. Ziola et. al. [19] demonstrates a simple leak location method using a very thin aluminum plate, in which the method utilizes the classical plate theory and cross-correlation technique. Given a priori knowledge of the wave speed in a certain frequency range, the method can properly locate the leaks in the thin plate. Their study is limited to very thin plates whose thickness is less than 2.5 mm.
Although a simple cross correlation method is marginally capable of locating the leaks, it provides unambiguous indications of a leak only if it satisfies the simplified assumption of a single mode of propagation with a non-dispersive (or frequency-independent) wave speed. In fact, the structure-bone noise in a plate-like structure, such as the outer skin of spacecraft, is evidently carried by multiple dispersive modes of plate wave propagation. In a dispersive medium, the structure-borne noise propagates with a different velocity at every frequency so that the cross correlation technique is not suitable to adequately provide an exact location of the leak source. The method provides much poorer results as the received signals become more dispersive. Examples of ambiguous indications in dispersive media are shown in References 20-23. In addition, the temporal shape of the propagating structure-borne noise does not remain in constant. Furthermore, the effect of the dispersion becomes more serious as the distance between the sensors at remote locations and the leak source increases. As a result, the cross correlation technique is not a viable option to locate leaks in dispersive media or structures.
In order to compensate the dispersion-related drawbacks of the cross correlation technique in source location, Rewerts et al. [20-21] have demonstrated a dispersion compensation method in a 1-dimensional structure for water-filled pipelines. This method isolates particular modes from the dispersed measurements as well as utilizing the advantages of the cross correlation technique. Steri et al. [22-23] have extended the work of Rewert, et al. and showed a robust mode-compensating leak location method in a highly dispersive structure, with a leak and sensors arranged in a collinear fashion. This method isolates the contributing modes of propagation and determines the compensation for the frequency dependence of the contributing mode wave speed. To adequately isolate the particular modes, it performs temporal and spatial Fourier transforms on the received signals. These works also emphasize that the signals need not be collected simultaneously for the cross correlations among several pairs of sensors, owing to the stationary property of the power spectrum of the cross-correlated signals.
Despite these works, significant problems remain in leak detection. What is needed is a sensitive and reliable means to locate an air leak in pressurized, habitable and long-endurance spacecraft. Such a method should be able to locate air leaks accurately with the requisite speed.